Question about unjoined/unconnected mesh/object
I think there are two questions here: 


Should the parts of my model be manifold? 
Is it OK for my model to comprise several parts, manifold or not? 


For all the reasons given in it, the first question has a nice answer here

As for the second, IMO,  wherever possible, model parts which are separate parts in the real world as separate parts in the model. Reasons not to do that include:


3D printing as one piece.
Needing a surface to behave well under (possibly unrealistic) deformation when animated
Economy. Simply reducing complexity and poly-count, modeling time.


But you often see people struggling to achieve good topology across regions of a model which could be separate surfaces with no harm done at all.